In order to operate internal combustion engines having at least two cylinders, the necessary injection of fuel can be made into an intake manifold assigned to the respective cylinder (intake manifold injection), or directly into a combustion chamber of the respective cylinder (direct injection). The injection is carried out regularly using an injection valve. Here, the opening duration of the injection valve correlates with the quantity of fuel injected. The relation between quantity of fuel injected and duration of opening of the injection valve can be reproduced by a valve characteristic curve that is valid for example for a particular fuel pressure at the injection valve. However, a valve characteristic curve can also depict the relation between quantity of fuel injected and some other control quantity of the injection valve (e.g., duration of an applied opening voltage).
To this extent, the quantity of fuel that is to be supplied or injected to the cylinder is in principle controllable via the opening duration, or the other control quantity, of the injection valve. Here, however, it is problematic that injection valves regularly have different properties depending on the particular, individual valve. These result for example from manufacturing tolerances. To keep the valve characteristic curve of an injection valve largely free of such individual variations means in most cases a high manufacturing outlay and therefore high production costs. For this reason, the valve characteristic curves of standardly used injection valves regularly show different curves for different individual units. In particular, there often occurs a deviation from a linear relation (assumed to be ideal) between the injection quantity and the duration of opening.
If the internal combustion engine is operated without measures for compensating the individual unit-dependent deviations of the various injection valves, this can result in an injection of different quantities of fuel into the different cylinders that are ignited one after the other. This can have a disadvantageous effect on smooth running, on the temporal course of the torque, wear characteristics, or exhaust gas emissions of the internal combustion engine.
German Patent Application No. DE 10 2005 051 701 A1 describes a method for operating an internal combustion engine in which the characteristic curve of the injection valve is adapted; i.e., deviations of the actual injected quantity from the target injected quantity are acquired and compensated. For this purpose, the overall injection is divided into a measurement injection and a basis injection, and the measurement injection is used to determine the deviation between the target and actual injected quantity. Here, the named deviation is detected in particular via a deviation of a lambda probe signal in the exhaust gas from a determined target value.
In such a procedure for correcting deviations between the valve characteristic curves of different injection valves, the problem can arise that the respective cylinder is operated with a non-optimal mixture due to the measurement injection. This occurs in particular when the assigned measurement injection takes place in an operating state of the injection valve in which there occurs a large deviation of the actual and target value of the injected quantity, i.e., a large deviation of the individual-dependent valve characteristic curve from a standard characteristic curve or from an assumed ideal characteristic curve. In this region, the measurement injection therefore cannot take place in a “mix-neutral” fashion, i.e., the fuel mixture ignited in the respective cylinder is modified by the measurement injection itself. This effect can occur in particular in an operating range having very short control durations of the injection valve (i.e., short opening duration), because in the case of small injected quantities the individual-dependent deviations of target and actual value of the injected quantity result in particularly large relative errors in the mix composition.
Therefore, the execution of a measurement injection regularly itself causes mix errors. Thus, the described procedure can have the result that some or all cylinders of an internal combustion engine are operated with a non-optimal mix, which in turn can result in an impairment of smooth running, fluctuations in torque, or worsening of the emission/wear properties.